The present invention relates to crystal growth, and, in particular, relates to the growth of bulk crystals of silicon-germanium.
Single crystals of Si.sub.1-x Ge.sub.x are of interest for a variety of applications from high speed microelectronics to thermo-photovoltaics. Many of these applications require deposition of thin films onto silicon substrates with a consequent strain due to lattice mismatch. Since bulk crystals of uniform concentration are not yet available, the growth of unstrained layers directly on the substrate is out of the question, and a comparison of the properties of strained layers with the intrinsic bulk properties is impossible. In fact, there has been little investigation of the physical properties of bulk SiGe alloys and their potential application for electronic and photonic devices. Despite the fact that silicon and germanium form a continuous solid solution, growth from the melt has had limited success in the past because solidification is non-congruent with the melt. Attempts at Czochralski growth and Bridgman growth techniques have shown that such conservative growth processes cannot produce crystals of constant composition. More recent studies have pointed to the floating zone technique in order to control the composition. While this method offers the possibility of containerless growth, it requires metered feeding of germanium granules into a silicon rod in order to control the growth composition. In the case of contained growth, an additional problem arises from the nucleation of multiple grains because of the melt sticking to the container. A solution to this problem was discussed in earlier work on directional solidification of silicon using calcium chloride. Both of these are obstacles to successful crystal growth, compositional uniformity and adherence of the melt.
Thus, there exists a need for a process for growing single crystals of silicon-germanium of uniform compositional quality.